The invention relates to a connecting device for connecting optical waveguides, in particular for connecting waveguides of different refractive index in a low-reflection fashion.
Glass fiber networks that are distinguished by high data transmission rates play a key role in telecommunication. Optical data telecommunications with the aid of glass fibers require amplifiers at regular intervals for this purpose. The previously used electronic amplifiers with electronic converters are increasingly being replaced in this case by optical amplifiers, in particular by optical fiber amplifiers. It is possible by using such optical devices to implement transmission rates that are higher by several orders of magnitude than the transmission rates that can be realized with the aid of electronic amplifiers.
However, there is the problem in this regard that the core refractive indexes of the optical amplifiers and of the generally used monomode fibers for data transmission differ markedly as a rule. The coupling of fibers of different refractive indexes is attended, however, by undesired effects owing to reflection losses, and by signal distortions and ghost signals owing to multiple reflections. Also particularly disturbing are reflections in or at surfaces of emergence of optical fiber amplifiers, where undesired resonances can occur inside the amplifier because of the reflections. The end faces of the waveguides can be bevelled in order to avoid instances of coupling into the amplifier, that are caused by reflection, and reflection losses, or at least to reduce them.
Such a coupling of waveguides is disclosed, for example, in European Patent EP 0 194 325, in the case of which the fibers to be coupled to one another are ground at the same angle. However, if the fibers have different core refractive indexes, when light is coupled into the second fiber this leads to an angular offset between the optical axis of the output fiber and the optical axis of the input fiber. Consequently, in the case of such an arrangement, the optical axis of the input fiber must run obliquely to the optical output axis. Such arrangements are, however, difficult to adjust and therefore expensive to fabricate.
Again, EP 0 858 976 A2 discloses a device for coupling optical fibers, in the case of which the end faces of glass fibers of different refractive indexes respectively have different lead or grinding angles. Again, in the case of this embodiment of a waveguide connector the glass fibers are coupled to one another in such a way that the end faces of the fibers are aligned parallel to one another. However, the result of this is that the optical axes of the two glass fibers are necessarily at an angle to one another. A substantial outlay on adjustment and mounting also results from these embodiments.
The present invention has therefore addressed the object of providing an improved coupling between waveguides of different refractive indexes. The aim here is not to restrict this solution to fiber amplifiers, but rather that it can be applied for surface waveguides or waveguides in three-dimensional space, for example.
This aim is already achieved in a most surprisingly simple way with the aid of a connecting device for connecting at least two optical waveguides having the features of claim 1.
In the case of the connecting device according to the invention for connecting at least two optical waveguides, a first optical waveguide is held relative to a second optical waveguide, an end face of the first waveguide running obliquely to the optical axis of the first waveguide, and an end face of the second waveguide running obliquely to the optical axis of the second waveguide. The refractive index of at least one light-guiding region of the first optical waveguide differs in this case from the refractive index of a light-guiding region of the second optical waveguide, and the above described problems of the prior art are avoided, since the end face of the first optical waveguide and the end face of the second optical waveguide are inclined such that the optical axis of the first and of the second optical waveguides are arranged substantially parallel to one another.
This particular arrangement of the waveguides permits an extremely simple design of the connecting device according to the invention, in the case of which the waveguides meet one another not obliquely, but in a straight line in the plug-in direction, additionally rendering possible low-reflection coupling of waveguides of different refractive indexes of the light-guiding regions.
The result of this is a lower space requirement by comparison with the coupling with the aid of fibers that meet one another obliquely, particularly also whenever a plurality of connections are to be arranged next to one another. Moreover, axial play between the waveguides, or a slight axial maladjustment is substantially less critical.
It is advantageous for the purpose of low-reflection coupling of optical signals into the second optical waveguide when, furthermore, the connecting device according to the invention is designed in such a way that a wave that is being guided in the first optical waveguide emerges, owing to refraction at the end face of the first optical waveguide, from the first optical waveguide at an angle a obliquely to the optical axis of the first optical waveguide, and enters the second optical waveguide obliquely to the second optical axis of the second optical waveguide and, owing to refraction at the end face of the second optical waveguide, propagates in the second optical waveguide substantially parallel to the optical axis of the second optical waveguide.
This has the advantage, furthermore, of thereby rendering possible a very precise fine adjustment of the fibers. Light exits more obliquely relative to the optical axis owing to the aligned or rectilinear arrangement of the waveguides relative to one another, and to the oblique arrangement of the end faces. Consequently, an axial displacement of the fibers relative to one another causes a displacement of the point where the signal is incident on the fiber, into which the signal is launched again, in the radial direction. However, this displacement is reduced by the factor of the sine of the angle xcex1 between the optical axis and the direction of light propagation in the gap between the waveguides. This factor renders an exact fine adjustment possible, since at small angles of the end face normal to the optical axis a relative displacement in the axial direction causes only a radial displacement of the point of incidence that is reduced by this sine factor.
In the connecting device according to the invention, the core of the first optical waveguide preferably has a different refractive index than the core of the second optical waveguide.
It is, moreover, advantageous for the transmission properties of the connecting device in accordance with the present invention when the spacing of the end face of the first optical waveguide from the end face of the second optical waveguide varies in the direction of the optical axis of the first optical waveguide along the direction perpendicular to this optical axis. Disturbing resonance phenomena can be reduced to a large extent by this arrangement. Consequently, ghost signals produced by the varying spacing, in particular by multiple reflection, are further damped.
The invention can be used with particular advantage for coupling optical waveguides with optical amplifiers that generally have a different refractive index than the waveguide for signal transmission. Consequently, in a practical way at least one of the two optical waveguides comprises an optically amplifying material in a practical way for such an embodiment of the connecting device.
The optical waveguide for the optical amplification can, in particular, also have a region doped with rare-earth elements.
In accordance with a preferred embodiment of the invention, the first optical waveguide comprises a fiber waveguide, and the second optical waveguide comprises an amplifier fiber doped with erbium.
For long transmission paths, in particular, quartz glass is particularly suitable as material for the first optical waveguide in the case, for example, of this embodiment, since it is distinguished by particularly low coefficients of absorption for the frequencies usually employed in optical signal transmission. Suitable inter alia for the second optical waveguide are glasses from a group which comprises bismuth oxide-containing glass, tellurite glass, germanium sulfide-containing glass and fluoroaluminate glass. These glasses constitute particularly suitable matrixes for optical amplifier media, in particular for rare earth ions.
In accordance with a further embodiment of the invention, it is provided that the normal to an end face of the first optical waveguide is inclined at 8xc2x0 to the optical axis of the waveguide. It is preferred in this case that the normal to an end face of the second optical waveguide is inclined at 21xc2x0 to the optical axis of the waveguide.
According to another embodiment, the normal to an end face of the first optical waveguide is inclined at 31xc2x0 to the optical axis of the waveguide, and in a preferred development the normal to an end face of the second optical waveguide is inclined, in particular, at 15xc2x0 to the optical axis of the waveguide.
It is expedient for the different inclinations of the end faces of the waveguides to have the effect that, at least in the region of the cores, the end faces have a minimum spacing from one another that is determined by the dimensions of the guides. Since the coupling losses can rise with increasing spacing of the waveguides from one another, in a preferred development at least one waveguide has one further end face such that the end faces thereof are at an obtuse angle to one another seen from outside. It is thereby possible for the waveguides to be brought closer to one another, and it is ensured that the coupling losses are substantially always minimized. It is therefore advantageous when the end faces of at least one of the two waveguides are arranged at an obtuse external angle to one another.
Thus, for example, it is provided in an embodiment in the case of which the waveguides have a plurality of end faces that the normal to an end face of the first optical waveguide is less than or equal to 15xc2x0, and a further end face is inclined at an angle of less than or equal to 21xc2x0 or 31xc2x0 to the optical axis of the waveguide.
It is proposed, furthermore, in particular that the normal to an end face of the first optical waveguide is inclined at 21xc2x0 to the optical axis of the waveguide, the normal to an end face of the second optical waveguide is inclined at 15xc2x0, and a further end face of the second waveguide is inclined at an angle of at least 21xc2x0 to the optical axis.
It is preferably provided in the case of a plurality of end faces of at least one waveguide that the normal to one of the two end faces is inclined at an angle of 21xc2x0 to the optical axis of the waveguide, the normal to an end face of the other one of the two waveguides additionally or alternatively preferably being inclined by 15xc2x0 to the optical axis of the waveguide.
The invention also provides that in the case of all embodiments of the connecting device the waveguides can also be coupled to one another via an adapter. The adapter can be used, for example, to adapt the refractive index of the interspace between the end faces, or to influence the shape and intensity distribution of the wave front of the light emerging from one waveguide, for example via a refractive index that varies in the volume of the adapter.
It is provided, furthermore, to design the adapter either with a further waveguide arranged in the latter, or with a channel that couples the waveguide cores to one another and preferably has reflecting walls, in order to reduce coupling losses. This channel can be designed as part of a photonic crystal, and can provide optical properties resembling a waveguide by means of its highly reflecting inner walls.
Since light emerging from one waveguide is propagated in the interspace between the two boundaries at an angle to the optical axis of the waveguide, and can cause an offset perpendicular to the optical axis when impinging on the further waveguide, it is also advantageous in such a case when the core of the first optical waveguide is laterally offset relative to the core of the second optical waveguide.
Moreover, it can be generally advantageous when the end face of at least one of the waveguides is not even. In particular, it is possible by appropriate selection of the shape of the end face to optimize for launching the surface of the phase front and the intensity distribution of the light upon impingement on the waveguide into which the signal is launched again. For example, one of the or a plurality of end faces of at least one of waveguides can be rounded, for example be designed in a concave or convex fashion, be uneven and/or curved. Furthermore, it is advantageous for optimum launching, in particular, when the end faces of the optical waveguides are designed in such a way as to permit the phase front to be preserved in a transition of an optical wave from the first into the second waveguide.
In order for the wave front to propagate in the direction of the optical axis again after launching in the waveguide, it is advantageous, furthermore, when the optical axis of the first waveguide, the optical axis of the second waveguide and the normal vectors to the first and the second end faces of the waveguides lie substantially in one plane. In the case of uneven end faces, the normal vector is to be understood in practical terms here as the normal vector at the location of the end face through which the optical axis of the waveguide runs.
A particularly effective suppression of reflections can be achieved at the waveguide ends when the end faces of the waveguides are coated with an antireflection layer. This also serves, in particular, to optimize the transmission of the arrangement. Moreover, the refractive index effectively acting at the end face can also be influenced or set with the aid of such an antireflection layer.